what length fan belt ??

Take a piece of twine or string and run it through the pulleys, then measure your twine or string for length.
 
Yes that will get you close --- when I do that get several belts -- like the next sizes smaller and next sizes larger as it is next to impossible to get the proper size the first time around . Your belt supplier should take back all unused belts and refund your money -- to be sure just ask.
 
I agree any parts store ?worth their salt? should be able to look up that by application. I was going post a link to Rockauto.com for your truck but they only go up to F500 and they show a bunch of different ones that include air pump and A/C etc.
 
Should have been wearing your panty hose. Take one of the legs and go around the water pump and crankshaft pulleys only. Tie a tight knot. Can get you pretty far.
 
P.S. Take a dress makers tape measure and run it around the outer edge of the pulleys. Gets pretty dang close. When you get 3verything fitted up take your Milwaukee magic marker and write the belt number under the hood or on the firewall.
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I've taken a piece of twine and tied it so it was tight. Then take the tied piece to an auto parts store and had them measure it on a belt "sizer". It worked, nobody laughed. They had seen it before!
 
Thanks guys for all the tips,, 391 takes a 44 inch belt , But i think a 43 would fit better.. my local farm store knew about which size ,. he gave me 4 belts 44-48 inches just like another poster suggested,.and i took back which ever would not fit.
 

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